Allen was born in Brooklyn, New York, USA, on 6 February 1878, to parents George Henry Sandison (1851–1934) and Charlotte D. Baxter (1850–1922). Her parents were Scottish immigrants. Her father was a religion writer and editor of several magazines and newspapers, including the Christian Herald and the New York Star. Allan had three sisters and two brothers.

For several years, she pursued a teaching career before obtaining her medical degree at the New York Medical College for Women in 1917, at the age of 39. Prior to attending medical school, she worked as a school teacher. She opened a private practice on Manhattan’s Morningside Drive soon after her graduation.

On 18 December 1931, at the age of 53, she married journalist Beecher Allen. Even after marriage, she was known professionally by her maiden name. Her husband passed away in 1949. After 25 years of practicing medicine, she closed her Manhattan office and transitioned to a career as a staff doctor for the New York Telephone Co. Throughout the 1940s, she conducted physical examinations for the company’s employees. Following her retirement from the telephone company, she established a rest home for elderly women in Nyack. She operated the home until she was in her mid-90s.

At the age of 103, she left her home in Nyack, New York, and moved into the Fountain West Health Center in West Des Moines, Iowa. When asked about the secret of her longevity, she said she never took medicine, she read the Bible and she conscientiously thanked the Lord. At the age of 110, she was reported to be frail and either unwilling or unable to communicate. The health center’s activity director noted that while her mind remained sharp, she struggled to articulate her thoughts, leading her to eventually give up trying to express herself.

Allen passed away in West Des Moines, Iowa, USA, on 23 October 1988, at the age of 110 years, 260 days.